The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 standard is a family of standards for wireless local area networks (WLAN) in the unlicensed 2.4 and 5 Gigahertz (GHz) bands. The current IEEE 802.11b standard defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Megabits per second (Mbps). The 802.11b standard uses direct sequence spread spectrum (DSSS) with a chip rate of 11 Megahertz (MHz), which is a serial modulation technique. The IEEE 802.11a standard defines different and higher data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band. The FCC has also approved a modified version of 802.11a to run in a licensed band near 6 GHz. It is noted that systems implemented according to the 802.11a and 802.11b standards are incompatible and were not designed to work together.
A new IEEE standard is being proposed, referred to as 802.11g (the “802.11g proposal”), which is a high data rate extension of the 802.11b standard at 2.4 GHz. It is noted that, at the present time, 802.11g is only a proposal and is not yet a completely defined standard. Several significant technical challenges are presented for the new 802.11g proposal. It is desired that the 802.11g devices be able to communicate at data rates higher than the standard 802.11b rates in the 2.4 GHz band. In some configurations, it is desired that the 802.11b and 802.11g devices be able to coexist in the same WLAN environment or wireless area without significant interference or interruption from each other, regardless of whether the 802.11b and 802.11g devices are able to communicate with each other. Thus, it is desired that 802.11g be backwards compatible with 802.11b devices. It may further be desired that the 802.11g and 802.11b devices be able to communicate with each other, such as at any of the standard 802.11b rates.
An impairment to wireless communications, including WLANs, is multi-path distortion where multiple echoes (reflections) of a signal arrive at the receiver. Other types of interferences, such as different and incompatible wireless signal types, may cause problems with WLAN communications. The Bluetooth standard, for example, defines a low-cost, short-range, frequency-hopping WLAN. Systems implemented according to the Bluetooth standard present a major source of interference for 802.11-based systems. Both the single-carrier systems and multi-carrier systems include equalizers that are designed to combat various types of distortion. The equalizers are typically designed to use the preamble to achieve good receiver acquisition. One proposal to implement 802.11g is a mixed mode configuration including a single-carrier segment with a preamble and header and a multi-carrier segment with a payload. The traditional multi-carrier system, however, was not designed to utilized the information obtained from a single-carrier preamble. Losing all information when transitioning from single-carrier to multi-carrier is not desirable in the presence of multi-path distortion or other types of interference.
There are also several potential problems with the signal transition between single- and multi-carrier signals, particularly with legacy equipment. The transmitter may experience analog transients (e.g., power, phase, filter delta), power amplifier back-off (e.g. power delta) and power amplifier power feedback change. The receiver may experience Automatic Gain Control (AGC) perturbation due to power change, spectral change, multi-path effects, loss of channel impulse response (CIR) (multi-path) estimate, loss of carrier phase, loss of carrier frequency, and loss of timing alignment.
A mixed waveform configuration for wireless communications was previously disclosed in U.S. Provisional Patent Application entitled, “Wireless Communication System Configured to Communicate Using a Mixed Waveform Configuration”, Serial No. 60/306,438 filed on Jul. 6, 2001, which is incorporated by reference in its entirety. The system described therein reused the equalizer information obtained during acquisition of the single-carrier portion of the signal. The technique provided continuity between the single-carrier and multi-carrier segments (e.g., orthogonal frequency division multiplexing or OFDM), which was achieved by specifying the transmit waveform completely for both the single-carrier and multi-carrier segments and specifying the transition. The waveform enabled continuity between the two signal segments, including AGC (power), carrier phase, carrier frequency, timing and spectrum (multi-path). It was contemplated that the signal would not have to be reacquired by the multi-carrier portion of the receiver since the information developed during the single-carrier portion (preamble/header) was valid and used to initiate capture of the multi-carrier portion. However, particular receiver architectures were not discussed.
A mixed carrier wireless architecture has been previously disclosed in U.S. Provisional Patent Application entitled, “Single-Carrier to Multi-Carrier Wireless Architecture”, Ser. No. 60/325,048 filed on Sep. 26, 2001, which is incorporated by reference in its entirety. The wireless architecture described therein is capable of communicating using the proposed mixed carrier waveform configuration. The term “mixed carrier” refers a combined signal with a single-carrier portion followed by a multi-carrier portion. The transmitter could be configured to operate in multiple operating modes including single-carrier, mixed carrier and multi-carrier modes. Furthermore, several receiver architectures were described that are configured to receive a mixed carrier signal and resolve the Baseband signals incorporated in the mixed carrier signal.
A Baseband transmitter and receiver architecture according to one embodiment of the prior disclosure achieves coherency across the single-carrier to multi-carrier transition by maintaining gain, phase, frequency, sample timing and Channel Impulse Response (CIR) from the single-carrier signal to the multi-carrier signal of a mixed carrier signal. In this manner, the signal does not have to be reacquired by the multi-carrier portion of the receiver since the information developed during the single-carrier portion is valid and used to initiate capture of the multi-carrier portion. Maintaining and accumulating information makes the signal much more robust in the face of common interferences experienced in wireless communications. A Baseband receiver architecture according to an alternative embodiment was also described that does not preserve the coherency across the transition, so that the multi-carrier portion of the receiver must completely re-acquire the signal after the transition. A multi-carrier preamble may be used for this purpose. Yet another non-coherent receiver embodiment was disclosed that utilizes selected information gained from the single-carrier portion of the waveform, such as any selected parameter associated with gain, phase, frequency or timing. Although the non-coherent architectures are less robust than the coherent configurations, the non-coherent options may be easier and cheaper to implement while remaining sufficiently robust to achieve a suitable communication system for many applications.
A technical challenge of the mixed carrier transmitters is rate changing either or both of the single-carrier and multi-carrier signals so that they may be combined in a coherent manner. Several rate changing techniques are described herein.